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            <h2 class="title"><a id="introduction"></a>Chapter 1. Introduction</h2>
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          <b>Table of Contents</b>
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          <dt>
            <span class="sect1">
              <a href="introduction.html#txnintro">Transaction Benefits</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="sysfailure.html">A Note on System Failure</a>
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          <dt>
            <span class="sect1">
              <a href="apireq.html">Application Requirements</a>
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          <dt>
            <span class="sect1">
              <a href="multithread-intro.html">Multi-threaded 
        <span>and Multi-process</span>
        Applications</a>
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          <dt>
            <span class="sect1">
              <a href="recovery-intro.html">Recoverability</a>
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            <span class="sect1">
              <a href="perftune-intro.html">Performance Tuning</a>
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      <p>
    This book provides a thorough introduction and discussion on transactions as
    used with Berkeley DB (DB). 
    
     
        
    It begins by offering a general overview to
    transactions, the guarantees they provide, and the general application
    infrastructure required to obtain full transactional protection for your
    data. 
  </p>
      <p>
    This book also provides detailed examples on how to write a
    transactional application. Both single threaded and multi-threaded <span>(as well as multi-process
    applications)</span> are discussed. A detailed description of various
    backup and recovery strategies is included in this manual, as is a
    discussion on performance considerations for your transactional application.
  </p>
      <p>
    You should understand the concepts from the
        <span>
                <em class="citetitle">Getting Started with Berkeley DB</em>
        </span>
        
        
     guide before reading this book.
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              <h2 class="title" style="clear: both"><a id="txnintro"></a>Transaction Benefits</h2>
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        <p>
        Transactions offer your application's data protection from
        application or system failures.  That is, DB transactions offer 
        your application full ACID support:
    </p>
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            <li>
              <p>
                <span class="bold"><strong>A</strong></span>tomicity
            </p>
              <p>
                Multiple database operations are treated as a single unit of
                work.  Once committed, all write operations performed under 
                the protection of the transaction are saved to your databases.
                Further, in the event that you abort a transaction, all write
                operations performed during the transaction are discarded.
                In this event, your database is left in the state it was in
                before the transaction began, regardless of the number or
                type of write operations you may have performed during the
                course of the transaction.
            </p>
              <p>
                Note that DB transactions can span one or more
                database handles. 
            </p>
            </li>
            <li>
              <p>
                <span class="bold"><strong>C</strong></span>onsistency
            </p>
              <p>
                Your databases will never see a partially completed
                transaction. This is true even if your application fails while there are
                in-progress transactions. If the application or system fails,
                then either all of the database changes appear when the
                application next runs, or none of them appear.
            </p>
              <p>
                In other words, whatever consistency requirements your application has will never be violated by DB.
                If, for example, your application requires every record to include an employee ID, and your
                code faithfully adds that ID to its database records, then DB will never
                violate that consistency requirement. The ID will remain in the database records until such a time as your
                application chooses to delete it.
            </p>
            </li>
            <li>
              <p>
                <span class="bold"><strong>I</strong></span>solation
            </p>
              <p>
                While a transaction is in progress, your databases will appear
                to the transaction as if there are no other operations 
                occurring outside of the transaction. That is, operations
                wrapped inside a transaction will always have a clean and
                consistent view of your databases. They never have to see
                updates currently in progress under the protection of another transaction. 
                Note, however, that isolation guarantees can be 
                
                 relaxed from the default setting.   See 
                <a class="xref" href="isolation.html" title="Isolation">Isolation</a>
                for more information.
            </p>
            </li>
            <li>
              <p>
                <span class="bold"><strong>D</strong></span>urability
            </p>
              <p>
                Once committed to your databases, your modifications will
                persist even in the event of an application or system failure.
                Note that like isolation, your durability guarantee can be
                relaxed. See <a class="xref" href="nodurabletxn.html" title="Non-Durable Transactions">Non-Durable Transactions</a>
                for more information.
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